Iron powder for making sintered iron articles



2,994,600 IRON POWDER FOR MAKING SINTERED IRON ARTICLES FriedrichHansen, 17 Rue Carteret, Geneva, Switzerland No Drawing. Filed Aug. 14,1959, Ser. No. 833,681 Claims priority, application Switzerland Sept. 1,1958 4 Claims. (Cl. 75-5) The present invention is concerned with aniron powder serving to make sintered iron articles characterized notonly by their good mechanical resistance but chiefly by their highresistance to wear and tear and by a minimum tendency to corrosion.

According to my invention, such products are obtained, in a remarkablemanner, owing to the fact that the iron powder intended to be used formaking sintered bodies contains, in admixture therewith, an iron powderanalogous but having undergone a preliminary thermal treatment by meansof a gas capable of giving E nitrogen, for instance ammonia.

If pure iron powder suitable for powder metallurgy is treated attemperatures ranging from 300 to 4-50 C. by means of a gas capable ofgiving off nitrogen at these temperatures, such as ammonia, it is foundthat the iron powder absorbs nitrogen without its hardness beingsubstantially changed. This method may be called soft nitriding.

Soft nitriding, as opposed to hard nitriding, that is to say hardeningby means of a nitriding operation, is not yet well known in the art. Thefollowing explanations will therefore be given concerning this method.

The term soft nitriding designates a thermal treatment of nonealloyedsteels, of technically pure iron and cast iron, by means of gases orsalts capableof off nitrogen at the temperature of treatment, thesurfaces of the above mentioned ferrous metals showing, after thistreatment, at most a very small increase of hardness, whereas theirresistance to Wear and tear has been considerably increased by thistreatment and their tendency to corrosion has been practicallyeliminated. The thermal treatment of the iron powder is advantageouslyperformed in an eddy flow of the gas, at temperatures ranging from 300to 450 C., the duration of the treatment ranging from 20 to 60 minutes,after which the iron powder contains, mainly in the form of solution,absorbed nitrogen. According to the nature of the iron powder that isused and to the conditions of heating, the treatment may give rise toseparations of iron nitride (Fe N).

If the powder thus treated is compressed so as to form articles of thedesired shape and if these articles are sintered in known manner byheating at suitable temperature, for instance 1050 C., a portion of theabsorbed gas which contains nitrogen escapes. The natural inferencewould be that, as a consequence of such a strong heating, the result ofthe thermal treatment of the iron powder has been cancelled.

Now, most unexpectedly, it has been found, as a result of a great numberof experiments, that this is not the case. On the contrary, the sinterediron articles that are obtained have the above mentioned typicalqualifies of ferrous metals having undergone soft nitriding.

On the other hand, it was found that the agglomeration by sintering ofthe iron powder particles had not the quality that can be obtained withthe usual iron powders. The cause of this defect is probably due to thefact that the excess of nitrogen containing gas escapes during thesintering process and interferes therewith.

Patented Aug. 1, 1961 The invention eliminates this defect by the use ofa usual iron powder suitable for the manufacture of sintered ironarticles, this powder being characterized by the fact that it contains,in admixture therewith, an iron powder analogous but which has beenpreliminarily subjected to a thermal treatment by means of -a gascapable of giving oif nitrogen, such for instance as ammonia.Experiments have shown that this addition may correspond to an amountranging from 6 to 35% by weight of the total mixture.

By using this mixture of iron powders, it has been found that thenitrogen containing gases which escape from the iron powder portion thathad undergone treatment, are absorbed either through diffusion orchemical combination during the sintering operation by the ironparticles that had not undergone treatment. In consequence the formationof an excess of free nitrogen which would interfere with the sinteringprocess is prevented.

The sintered iron articles made by means of the powder according to thepresent invention have, in addition to a good mechanical resistance, ahigher resistance to wear and tear, whereas their hardness is reduced;furthermore, they have a good deforrnability in the cold state and theirtendency to corrosion is extremely low.

The metallurgical treatment of the new iron powder according to thepresent invention does not call for any modification in the knownmethods. The ratio of the amount of powders treated by means of a gascapable of giving off nitrogen to the amount of untreated iron powderwhich, as a rule, ranges from 6 to 35 percent by weight of the totalmixture, is advantageously determined experimentally in accordance withthe require ments to be complied with by sintered iron articles to bemanufactured. The amount of treated iron powder to be addedadvantageously ranges, as an average, from 8 to 12 percent by weight ofthe total mixture. The addition of copper powder to the iron powder, inorder to reduce shrinkage during sintering, may be lower, when using theiron powder according to the invention, than in the case of the knowniron powders used for sintering. This addition of copper powder may bemade advantageously by chemical coppering, or by coppering by means ofvapors, of the treated iron powder used as addition in the mixture. 1

What I claim is:

1. In a process of producing sintered iron articles, the steps offorming a mass of substantially pure iron powder of suitable characterfor sintering, contacting a portion thereof constituting about 635% byweight of the mass with nitrogen at a temperature of about 300-450 C. tosoft nitride the portion, and admixing the soft nitrided portion withthe remainder of the mass.

2. A process according to claim 1 wherein about 8-12% by weight of themass is brought into contact with nitrogen.

3. A process according to claim 1 wherein the portion is brought intocontact with nitrogen for about 20-60 minutes.

4. A composition for making sintered iron articles consistingessentially of substantially pure iron powder of suitable character forsintering, about 6-35 by weight of the powder being soft nitrided.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN A PROCESS OF PRODUCING SINTERED IRON ARTICLES, THE STEPS OFFORMING A MASS OF SUBSTANTIALLY PURE IRON POWDER OF SUITABLE CHARACTERFOR SINTERING, CONTACTING A PORTION THEREOF CONSTITUTING ABOUT 6-35% BYWEIGHT OF THE MASS WITH NITROGEN AT A TEMPERATURE OF ABOUT 300-450*C. TOSOFT NITRIDE THE PORTION, AND ADMIXING THE SOFT NITRIDED PORTION WITHTHE REMAINDER OF THE MASS.